The present invention relates to a method and apparatus for restoring the tap hole of a steelmaking converter to a proper dimensional size after same has become worn due to use. In steel converters, particularly those which are used for refining by the oxygen lance process in which oxygen is blown into a molten bath of steel, the tap hole is, as a general rule, positioned in the upper half of the side wall of the converter. By tilting the converter, the molten charge therein will thereafter flow in a molten state from the reactor through the tap hole.
Such tap holes are formed generally by use of a so-called tap hole brick which is secured in a recess in the brickwork of the converter with the aid of granular or powdery refractory substance applied by ramming (monolithic lining, stampfmasse or pise'). The tap hole brick employed therein consists of a cylindrical brick provided therein with a central bore, such bricks generally being made of magnesite. After the converter has been tilted, the molten steel flows out through the central bore in the tap hole brick, the time required to pass the molten steel through the tapping hole depending primarily on the diameter of the central bore in the tap hole brick.
As a result of the molten steel flowing out of the formed tap hole, there will be an erosion of the wall of the hole and thus the diameter of the tap hole will gradually increase in size as a result of such operation. The shape of the tap hole also may change so as to deviate from the normally cylindrical shape of the hole. Thus, after the tap hole has become too enlarged, the tap hole becomes unsuitable for further use. This wear and erosion of the tap hole of the converter is highly undesirable since it will result in too short a tapping time, this being the consequence of a too wide or enlarged tap hole in the converter through which the molten metal will flow.
The fact that a too short tapping time will occur will also mean that a period of time will arise which will be too short for uniformly dissolving alloying materials into the molten steel which is generally added after the beginning of the tapping operation. This operation, resulting in too short a tapping time, is undesirable from a metallurgical standpoint. Another drawback resulting from a too wide or an enlarged tap hole is that the wall of the tap hole brick oftentimes becomes too thin and thus the wall thereof can break through or otherwise be damaged at a given moment. Therefore, from an operational standpoint, after the tap hole has reached a certain critical diameter due to erosion from the molten metal flowing therethrough, the tap hole brick should be repaired to restore the desired dimension to the tap hole or, otherwise, replaced.
As a result of the repairing methods that have been employed heretofore, it was found to be generally desirable to postpone the repair of a damaged tap hole in a converter as long as possible. One practice that has been employed heretofore to minimize the problem of the tap hole erosion has been that in the initial operation of the converter, to use a tap hole brick having a central bore therein which is smaller than the desired opening. Such practice had the advantage that only after several charges have passed through the tap hole, the desired critical upper dimensional limits of the diameter of the tap hole would be reached.
A typical repairing procedure for repairing a worn tap hole that has been employed heretofore is that when the repairing is to occur, the converter is shut down and then placed in a tilted position. A mold is then positioned from the outside of the converter into the damaged tap hole. Such a mold comprises a core which extends into and through the damaged tap hole and which has an outer diameter which corresponds to that of the desired inner diameter of the tap hole after the repair thereof has been made. After insertion of the mold into the tap hole, liquid repair material is then cast, with the aid of long handled spoons or ladles, through the large mouth or filling opening of the converter and into the annular space left free between the core of the mold and the wall of the tap hole to be repaired. The repair material contains a liquid dispersion or mixture of refractory material and a suitable binder which, after evaporation of the liquid therefrom, constitutes the refractory layer which is built up in the clearance or annular space between the core of the mold and the wall of the tap hole to be repaired. The mold is then removed from the repaired tap hole, after which the tap hole has thus been restored to its original dimensions and is in a condition for use with a number of charges. The positioning and removing of the mold in the worn tap hole is a time consuming operation and the manipulation with the long handled and heavy spoon or ladle containing the repair material before the hot converter mouth by personnel positioned before the very hot converter mouth is particularly undesirable to the worker from an operational and health standpoint. Therefore, the repairing of the tap hole in the manner above-described is a reason for postponing the operation for as long as possible.
By way of an illustration of the mold repairing procedure above-described, a typical converter is provided which is suitable for steel charges up to about 300 tons, with the converter having a bore diameter for the tap hole brick of about 150 mm. The normal tapping time for a converter having a tap hole of this dimension is about 11 minutes, which tapping time, however, after 60 to 70 charges have been discharged from the converter through the tap hole, has been reduced to about 5 minutes. The eroded tap hole must then be repaired to restore the converter operation to its normal state by having the bore size of the tap hole restored to its original dimension. The repairing of the tap hole will then be carried out in the manner hereinbefore described utilizing the mold procedure, after which the tapping time will be again about 11 minutes. After each repairing, however, it is possible to tap only about 20 charges before the tapping time of the converter has been reduced again to 5 minutes and the tap hole again repaired.
The just-described method of operating the converter and of repairing the tap hole after same has become worn, by utilizng a mold, is generally undesirable for various reasons. For example, a large number of charges will be tapped from the converter with long tapping times existing, a result of which is that there will be a corrosive attack on the converter wall lining by the slag floating on top of the steel. This fact is particularly undesirable since there will be a damaging effect on the lining since the attack will occur for a relatively long period of time. Therefore, it will be found desirable to restrict this so-called time of attack on the converter wall lining by the slag as much as possible. The desirability for minimizing this period of contact is that the attack time on the converter lining by the slag will also be determinative of the operational life of the refractory lining of the converter. This means that long tapping times and, therefore, long aggression or attack periods of time on the converter lining by the slag will result in the converter, after a fewer number of charges than desired have been made, having to be taken out of operation in order to be relined and provided with new bricks. Therefore, not only is the availability of the converter decreased as a result of the attack on the lining but, in addition, the costs of repair will result in a decrease in the production rate of the steel and a corresponding increase in the cost of the steel.
Another drawback with this prior method of repairing an eroded tap hole utilizing a mold is that, on the average, the longer tapping times decrease the availability of the converter, which will, in turn, result in a lower steel production per converter. Yet another drawback with this procedure is that the metallurgical control of the charge is quite difficult to achieve when the tapping times of the converter are found to vary considerably. Still another material drawback utilizing the mold repairing procedure is that the operating personnel must operate, when repairing the worn tap hole, quite close to the mouth of the converter, utilizing a bucket-shaped spoon having a long shank having a length of about 8 m. with the bucket-shaped spoon, when filled, weighing about 60 kg.
There have been other procedures that have been employed heretofore commercially or proposed for use in the repairing of eroded or worn tap holes in a converter. All of such prior procedures have been predicated on the philosophy that the tap hole should remain in use as long as possible even though it may become badly eroded and quite enlarged. When the tap hole has reached a state of erosion that it is impossible to use further, the converter will then be shut down and the tap hole will then be completely rebuilt or the tap hole brick replaced. Obviously, such a procedure is economically and operationally undesirable even though the shutdown period may be for only a short time.
In the first instance, the shutting down of the converter stops the steel production completely. Secondly, if the shutdown is for only a short period of time, problems are still created. For example, temperature is one of the most critical factors in steelmaking and a cooling thereof, even for a short time, will impair the steelmaking production.
Another critical factor in steelmaking production is that the tapping time of the converter will be materially affected by a complete shutdown of the converter for even a short period of time or by a gradual and extensive enlargement of the tap hole. The tapping time of a converter is a very critical factor in a steelmaking operation. The reason for this is that the tapping time and the tapping speed determine factors such as temperature of the tapped steel, carbon content and efficiency of additions of alloying materials which will effect the ultimate end product. Also, if the steelmaking process is controlled by a computer program, which in modern steelworks is not unusual, the tapping time is one of the essential parameters in the steelmaking procedure. If the tapping time varies within a wide range, this causes the outcome of the computer control to be very uncertain. For that reason, it is of paramount importance that the diameter of the tap hole remain as constant as possible during all tapping operations, which would not be the case if the tap hole repairs are carried out by previously employed and proposed procedures.
To sum up, with regard to the prior procedures that have been employed and proposed for repairing damaged tap holes heretofore, such procedures permit the steel converter to be used until the tap hole has become badly damaged, after which the steel converter is shut down for restoration of the tap hole. The repair of the badly damaged hole is achieved by use of molds or forms, jugs and the like or by slowly moving a housing through the tap hole opening to discharge the repair material against the damaged wall, or by use of other apparatus which will enable a worker to build up the tap hole wall. The shutting down of the converter to the extent required to effect the repair of the tap hole is obviously, for the reasons given hereinbefore, both operationally and economically undesirable.